Radio signal receiving system



W. R. KOCH RADIO SIGNAL RECEIVING SYSTEM March 30, 1937.

Filed Feb. 28, 1934 f ww Patented Mar. 30, 1937 UNITED STATES PATENT OFFICE RADIO SIGNAL RECEIVING SYSTEM Delaware Application February 28, 1934, Serial No. 713,254

10 Claims.

The present invention relates to radio signal receiving systems and more particularly to intermediate frequency or band pass amplifiers for use in such receiving systems.

It is an object of the invention to provide an improved band pass amplifier circuit.

It is a further object of the invention to provide an improved intermediate frequency amplifier for superheterodyne receivers and the like.

It is a still further object of the present invention to provide means for improving the selectivity of an amplifier channel in a superheterodyne receiving system, and to prevent interfer` ence from signals in adjacent channels.

In a radio receiving system wherein an intermediate frequency or b-and pass amplifier is ernployed, it is often difficult to secure suficient discrimination against adjacent channel signals. In accordance with the invention, a balanced amplifier circuit may be provided in a portion of the receiving system, such as the intermediate frequency amplier, and may be coupled between electric discharge or vacuum tube amplifier devices therein, to greatly reduce the interference from adjacent channel signals. The balanced circuit may be of the bridge type having frequency discriminating means in one arm thereof, with suitable balancing, electrical elements in an adjacent arm.

The bridge type of circuit of the present invention is balanced at the frequency of the adjacent channels. For example, with Vthe usual intermediate frequency amplifier adapted to pass signals at substantially 175 kilocycles, the frequency discriminating elements in the bridge circuit are tuned or adapted to balance the circuit at 165 and 185 kilocycles.

The invention will, however, be better understood from the following description when considered in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Figure l is a schematic circuit diagram of a superheterodyne receiving system provided with an intermediate frequency amplilier embodying the invention; and

Figures 2 and 3 are similar circuit diagrams showing modifications of the intermediate frequency amplier circuit of Figure 1, and of the invention.

Referring to Figure l, the rectangle indicated at 1i represents the usual radio frequency amplier, oscillator and first detector portions of a superheterodyne receiver and the rectangle 6 55 represents the usual second detector and audio frequency amplifier, between which is connected an intermediate frequency amplifier 1. The radio frequency amplifier portion receives signals from any suitable source, such as an antenna system 8, and the audio frequency amplier por- 5 tion is connected with a suitable output or loudspeaker device indicated at 9.

The invention is, however, concerned with the amplifier portion of the receiving system to which 10 a frequency discriminating balanced circuit or network may be applied, and as this is the intermediate frequency amplier, in the present example, further consideration of the other elements of the receiving system is believed to be unnecessary. 15

The intermediate frequency amplifier includes a plurality of cascade connected electrical discharge amplifier devices indicated at I0 and II, and is provided with input and output transformers or coupling devices indicated at I2 and I3, respectively. The amplifier devices I0 and II l may be of the vacuum tube or any suitable type, and in the present example are shown as being of the screen grid radio frequency pentode type such as the RCA- 58 tube.

Interposed between the tubes Illand II is an interstage coupling transformer I4 having a tuned primary winding I5 connected with the output electrode IB of the device III and having a tuned secondary winding I1 connected through a coupling network I8 with the input or control electrode I9 of the device II.

The coupling transformers I2, I3 and I4 represent any suitable amplifier coupling devices, and in the present example are all tuned broadly to substantially the same frequency band which includes the desired intermediate frequency in the present example. With an ampliiier of this character such as is adapted for conveying signals within a predetermined frequency band, interference from adjacent channel signals may be met in certain localities where several strong broadcasting channels may be closely adjacent to each other.

In accordance with the invention, the terminals of the tuned input circuit of which the secondary I1 is a part, are connected in balanced relation to the opposite input terminals 2l! and 2| of a balanced bridge or coupling network I8. The output terminals 22 and 23 of the bridge circuit are connected respectively to the control grid I9 and to a cathode return lead 24 whereby the output terminals of the bridge are connected between the control grid and cathode of the device II,

The coupling network or bridge circuit includes a frequency discriminating arm 25 between the terminals 2| and 22, and a balancing arm 2S between the terminals 20 and 22. 'I'he remaining arms of the bridge are provided with equal resistors or other suitable impedance devices 21 and 28 connected, respectively, between the terminals 20 and 23 and the terminals 2| and 23.-

The frequency discrimin-ating side or arm 25 of the circuit includes a pair of piezo electric crystals 29 and 30 connected in parallel, each of said crystals being responsive in an adjacent channel frequency such as and 185 kilocycles, respectively, in the circuit shown.

In the balancing side or arm 26 of the circuit, there are provided a variable impedance or resistor 3l and a variable condenser 32, also in parallel. These devices are for the purpose of balancing the capacity of crystal holders and the equivalent resistance of the crystals.

The circuit arrangement is such that the bridge network is balanced at the interference frequency above and below the selected intermediate frequency such as kilocycles. With this arrangement, the output terminals 22 and 23 do not receive signals at these frequencies and therefore the interference is eliminated from the adjacent channels. Since the circuit is unbalanced at the chosen intermediate frequency, signals appear between the output terminals 22 and 23 and are applied to the control grid I9 of the following amplifier device il.

It will be seen that the resistors or impedance f elements 21 and 28 provide in effect a center tap to ground or cathode for the balanced input circuit, which includes the secondary winding l'l and the shunt tuning condenser indicated at 33. The circuit is not otherwise connected to ground.

Other suitable center tap means may be providedv for the balanced input circuit, and it will be seen further that the tuned input circuit forms a part of the coupling network, along with the balanced bridge circuit I8. In the circuit shown, the balanced bridge circuit or network I8 also provides the input circuit for the device I l.

Other suitable frequency discriminating means may be provided in the bridge network, and for a modification of the circuit shown in Figure 1, attention is now directed to Figure 2 in which the same reference numerals are used to designate like parts.

In Figure 2, the intermediate frequency amplier devices I0 and l l are coupled .through a suitable interstage coupling transformer 34 which is provided with a tuned secondary winding 35 having a center tap 36 through which the coupling network is returned to the ground or cathode return lead 24. In this manner, the center tap resistors are eliminated and the secondary 35 on either side of the tap 36 forms two of thev adjacent arms or sides of the bridge circuit.

The remainder of the bridge circuit includes two frequency discriminating tuned networks or circuits provided by series connected impedances or inductances 37 and 38 and tuning condensers 39 and 40, respectively. This arrangement provides two parallel connected series resonance circuits as one arm of the entire bridge network or circuit, the latter being designated generally by the reference numeral 4I.

The input terminals of the bridge circuit are indicated at 42 and 43, while the output terminals, include the center tap terminal 36 for the secondary winding 35 and a terminal 44 to which the grid i9 of the device Il is connected.

The remaining leg of the bridge circuit between terminals 42 and 44 comprises a balanced variable inipedance or resistor 45 and a balancing variable condenser 43. These are arranged in parallel to each other and are for the purpose of balancing the inherent impedance, or resistance and distributed capacity of the series resonance circuits in the adjacent arm of the bridge circuit.

With this arrangement, the signals received through the amplifier are applied to the terminals 42 and 43 of the bridge network and if outside of but adjacent to the selected intermediate frequency, are balanced by the circuit, through the two series resonance circuits in the output arm thereof. This is for the reason that the impedances of the series resonance circuits are substantially the direct current impedance of the elements which is balanced by the variable irnpedance or resistor 45.

The bridge network is provided with a center tap 33 on the input coupling tuned circuit. The center tap may be provided by other suitable means such as a pair of series connected shunt condensers 4l and 48, as shown in Figure 3 as applied to the circuit of Figure 1, in lieu of the series connected resistance or impedance elements 21 and 28. The circuit of Figure 3 is otherwise the same as that of Figure 1 and like parts throughout bear the same reference numerals. In this circuit the tuned bridge network is oating and accordingly grid bias potentials are supplied to the grid I9 through a grid leak resistor 49 connected between the cathode return lead 24 and the grid or the terminal 22.

It will, therefore, be seen that any suitable means may be provided for center tapping the balanced coupling circuit provided between two electrical discharge or vacuum tube amplifier devices in an amplifier of the band pass type and that the coupling network includes a bridge circuit wherein one leg of the bridge, such as an output leg is provided with parallel connected frequency discriminating means, tuned above and below the preferred or selected frequency, to balance the bridge circuit at the signal frequencies to be discriminated against. The circuit arrangement has the advantage of simplicity and balanced arrangement of few circuit elements.

Having shown the balanced bridge circuit interposed between two cascade electric discharge devices, it will be appreciated that such a balanced bridge network may be provided between other devices, and in other band pass amplifiers or between two band pass circuits in any suitable manner, provided, however, that the output terminal of the network is connected between two arms of the bridge, one of which includes the frequency discriminating means, and the other of which includes balancing impedance elements for neutralizing the impedance of the frequency discriminating means in the opposite or adjacent leg of the circuit.

I claim as my invention:

l. In an electric signal amplier, a signal conveying circuit, means in said circuit for tuning said circuit to receive and convey signals at and about a predetermined frequency in a relatively narrow band, and means connected with said circuit providing a bridge output connection therefor balanced at interfering signal frequencies differing from above and below but adjacent to said frequency band to permit the ow of signals through said output connection within said frequency band to the exclusion of signals at said adjacent frequencies.

2. In an electric signal amplifier, a signal conveying circuit, means in said circuit for tuning said circuit to receive and convey signals at and about a predetermined frequency in a relatively narrow band, means connected with said circuit providing a bridge output connection therefor, and means comprising a pair of parallel connected piezo-electric crystals responsive to interfering signals at frequencies differing from above and below but adjacent to said frequency band for balancing said interfering signal bridge connection at said frequencies to permit the flow of desired signals through said output connection within said frequency band to the exclusion of interfering signals at said adjacent frequencies.

3. In a radio receiving system, signal selecting means, a signal conveying circuit for selected signals, means in said circuit for tuning said circuit to receive and convey signals at and about a predetermined frequency in a relatively narrow band, means connected with said circuit providf ing a bridge output connection therefor, and

means in said bridge connection for balancing the same at frequencies differing from above and below but adjacent to said frequency band to permit the flow of selected signals through said output connection within said frequency band to the exclusion of interfering signals at said adjacent frequencies, comprising a pair of parallel connected series resonant circuits in one arm of said bridge connection, one of said circuits being responsive to signals at one of said adjacent frequencies and the other of said circuits being responsive to signals at the other of said adjacent frequencies.

4. In an intermediate frequency amplifier for a modulated intermediate frequency carrier wave and side bands thereof, means providing a tuned signal circuit responsive to a frequency band including said side bands, a signal output terminal, means for connecting the terminals of said circuit in balanced relation to said output terminal, said means comprising frequency discriminating elements connected in parallel between said output terminal and one of said circuit terminals, said elements being tuned to frequencies differing from above and below but adjacent to the carrier intermediate frequency and said side bands to which said circuit is tuned, balancing impedance elements connected between said output terminal and the other terminal of said tuned signal circuit whereby the connection provided by the first named means is balanced at said differing frequencies, and means providing an electrical center tap connection for said circuit.

5. In an amplier of the band pass type for high frequency signals in a predetermined frequency band between two adjacent signal channel frequencies, an electrical bridge network comprising a pair of parallel connected signal frequency discriminating means in one arm thereof, circuit balancing means therefor in a second leg of said bridge network, an output terminal connected between said arms, and a signal input circuit connected to the remaining terminals of said arms, said input circuit being tuned to said predetermined signal frequency band, and said frequency discriminating means being tuned to frequencies representing interference signals in said adjacent signal channels differing from above and below but adjacent to said nrst named frequency band, to balance said network at said differing frequencies.

6. In an intermediate frequency amplifier, the combination with an interstage coupling device responsive to a predetermined signal frequencf,7 and interfering adjacent signal channel frequencies, of an electrical bridge circuit having input terminals connected with the terminals of said coupling device, said device being tuned to the predetermined intermediate frequency and side bands thereof, means providing an output terminal for said bridge circuit, frequency discriminating means connected with one of said output terminals in one arm of said bridge circuit, means in a second arm of said bridge circuit connected with said output terminal providing resistance and capacity for electrically balancing the resistance and capacity of said frequency discriminating means.

7. In an intermediate frequency amplifier, the combination of an interstage coupling device responsive to a predetermined signal frequency and interfering adjacent signal channel frequencies, an electrical bridge circuit having input terminals connected with the terminals of said coupling device, said device being tuned to a predetermined intermediate frequency and side bands thereof, means providing an output terminal for said bridge circuit, frequency discriminating means connected with one of said output terminals in one arm of said bridge circuit, means in a second arm of said bridge circuit connected with said one of said output terminals providing resistance and capacity for electrically balancing the resistance and capacity of said frequency discriminating means, said frequency discriminating means including a pair of parallel connected piezo-electric crystals each tuned to an adjacent channel frequency for said circuit.

8. An intermediate frequency amplifier comprising, in combination, an interstage coupling device, an electrical bridge circuit having input terminals connected with the terminals of said coupling device, said device being tuned to a predetermined intermediate frequency carrier wave and side bands thereof and admitting portions of adjacent channel signals, means providing an output terminal for said bridge circuit, frequency discriminating means connected with one of said output terminals in one arm of said bridge circuit, means in a second arm of said bridge circuit connected with said one of said output terminals providing resistance and capacity for electrically balancing the resistance and capacity of said frequency discriminating means, said frequency discriminating means including a pair of series resonant circuits connected in paraliel and each tuned to an adjacent channel signal.

9. In a superheterodyne radio signal receiver, an intermediate frequency amplifier circuit including an electrical bridge network, a separately tunable signal circuit coupling device responsive to an intermediate frequency band including adjacent signal channel interference, said device being connected with the input terminals of said network, a signal output circuit connected with the output terminals of said network, means in one of the arms of said network for balancing said network at signal frequencies differing from above and below but adjacent to the predetermined intermediate frequency and side bands thereof, and means in a second arm of said bridge network for balancing the inherent impedance of said first named balancing means.

10. In an intermediate frequency amplifier, a

signal conveying circuit, said circuit including tuning elements whereby it is tuned to resonate about a predetermined intermediate frequency, said circuit further including two bridge arms, one of which comprises a pair of piezo-electric crystals responsive to signals at frequencies above and below but adjacent to said predetermined intermediate frequency and the side bands thereof, and means other than said bridge arms pro- Viding an electrical center tap connection for said circuit.

WINFIELD R. KOCH.

. CERTIFICATE CF CORRECTION.

Patent Noe 2,075,526. March 5o, 1957,

WINFIELD R. KOCH.

It is hereby Certified that error appears in the printed specification of the above numbered patent requiring Correction as follows: Page 2, first Column, line 5, for the word "with" read by; page 5, first Column, line lll., claim 2,' strike out "interfering signal" and insert the same after "said" in line l5, same claim; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the oase in the Patent Officeo Signed and sealed this 15th day o f February, A; Du 1958c Henry Van Arsdale (Seal) y Acting Commissioner of Patents l 

